Radio Frequency Power Amplifiers Adapted For Low Field Magnetic Resonance Imaging

Magnetic Resonance Imaging (MRI) is a reliable and established minimally invasive imaging technique that can provide diagnostically relevant information about the internal structures of the human body. While the basic design of the new MRI scanners is not much different from when they were first designed a few decades ago, finding new ways to modify these big, power-hungry, expensive and complex systems is becoming more and more essential. One of the ways of removing the restrictions that conventional MRI systems have is making them low field. This leads to lighter, smaller, simpler and less expensive MRI scanners that can potentially become portable. Once they are portable, MRI scanners can have various applications ranging from being used in emergency and operating rooms to being taken to remote areas and even outer space. The Space MRI Lab at the University of Saskatchewan focuses on building prototypes of portable MRIs for monitoring astronaut health by using TRansmit Array Spatial Encoding (TRASE). TRASE is an innovative MRI method that operates without relying on noisy, heavy and complex gradient coils. In TRASE, the spatial encoding happens based on the phase gradients of the transmit radio frequency (RF) magnetic field. TRASE-based MRI scanners have specific requirements. One of those requirements is RF power amplifiers (RFPAs) with high-power RF output, high duty cycle and fast switching times. These characteristics are important for achieving maximal TRASE MRI resolution. However, since no commercially available RFPA with these specifications exists, it was necessary to build RFPAs customized for TRASE applications. A class A/B Ham radio power amplifier design was modified to be more compatible with TRASE-based MRIs. As part of this thesis, two of these RFPAs were constructed at the University of Saskatchewan Space MRI Lab. The RFPAs were assembled to be used with the Merlin MRI, an ankle-sized portable MRI tested in zero-gravity which uses TRASE. Fortunately, both of the RFPAs showed expected results at the testing stage and have since been integrated with the Merlin MRI. Details of the assembly work are presented in this thesis